This invention relates to a dew sensor for detecting deposition of dews, that is, dewing, through a change in electric resistance.
According to the so far known methods, deposition of dews is detected by (1) optical changes in light reflection or water absorption spectra, (2) changes in resonance frequency of piezo-resonator, or decrease in Q, (3) changes in capacity, or (4) changes in electric resistance, among which the method (1) requires a highly precise optical system which is hard to carry and expensive, and is not readily applicable to the domestic appliances and the methods (2) and (3) have a complicated detection electric circuit, whereas the method (4) based on changes in electric resistance has been regarded as most preferable because of the relatively simple detection circuit.
The method (4) has two types, i.e. an alternating current type and a direct current type, depending upon the species of a power source for the contemplated detection circuit, and also has an electric resistance-increasing type [for example, National Technical Report Vol. 24, No. 3, June (1978)] and an electric resistance-decreasing type [Japanese patent application Kokai (Laid-open) No. 18947/80] with increasing humidity.
In the case of a dew sensor of alternating current type, an electrolyte such as a chloride or a phosphate is used in the humidity-sensitive layer. However, such dew sensor has a more complicated circuit than that of direct current type, particularly when it is applied to a small, light appliance, for example, a portable VTR, and thus is not preferable for such use.
On the other hand, in the case of a dew sensor of direct current type, (a) conductor powder, (b) semiconductor powder or (c) insulator powder is used together with organic polymer in the humidity-sensitivie layer, where the powder is dispersed in the organic polymer or is in mixture of the organic powder [said Japanese patent application Kokai (Laid-open) No. 18947/80 and National Technical Report Vol. 24, No. 3 June (1978)], and thus has such a disadvantage as slow response to dewing detection owing to the dispersed or mixed state.
A dew sensor of resistance-increasing type with increasing humidity has a low resistance at the ordinary relative humidity (95% RH or lower) such as 10 k.OMEGA., and the humidity-sensitivie layer is liable to generate the Joule heat, resulting in deterioration of the sensor. It has such a further disadvantage that the applicable voltage is limited to less than 0.8 V.
The cylinder of VTR is most susceptible to dewing due to a sudden change in temperature, etc., and once dews are formed on the cylinder, such troubles as sticking and winding of a tape to and around a cylinder, etc. are liable to take place, and thus it has been keenly desired that a dew sensor can quickly and sharply detect dewing, that is, without any substantial change in resistance in a low relative humidity range, in other words, with a sharp change in resistance in a RH range of 95-100% and a quick response to dewing. The sharp change in resistance and quick response to dewing can eliminate maloperation of a dew sensor. The conventional dew sensors have hardly satisfied these requirements.